1. Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic system, and more particularly relates to an image forming apparatus such as a copying machine, a printer, or a facsimile.
2. Related Background Art
Conventionally, an image forming apparatus, such as a copying machine, a printer, or a facsimile, that uses an electrophotographic system generally includes an electrophotographic photosensitive member (photosensitive member) functioning as an image bearing member that is of rotary drum type, an electrifying device that performs electrification processing so that the photosensitive member is uniformly electrified to have a predetermined polarity and potential, an exposing apparatus functioning as an information writing means for forming an electrostatic latent image on the photosensitive member subjected to the electrification processing, a developing apparatus that visualizes the electrostatic latent image formed on the photosensitive member as a developer image (toner image) using toner functioning as developer, a transferring apparatus that transfers the toner image from the surface of the photosensitive member to a recording material such as paper, a cleaning device that cleans the photosensitive member surface by removing toner (residual developer, transfer residual toner) residing on the photosensitive member after a transferring step although the amount of the residing toner is small, and a fixing apparatus that fixes the toner image on the recording material. The photosensitive member is repeatedly subjected to an electrophotographic process (an electrifying step, an exposing step, a developing step, a transferring step, and a cleaning step) and is used to perform image forming.
Toner residing on the photosensitive member after the transferring step is removed from the surface of the photosensitive member by the cleaning device, is accumulated in the cleaning device, and becomes waste toner. However, from the viewpoint of environmental protection and effective use of resources, it is preferred that the generation of such waste toner is prevented.
Accordingly, there is used an image forming apparatus that returns transfer residual toner (so-called waste toner) collected by the cleaning device to the developing apparatus for reuse.
Also, there is used an image forming apparatus using a cleanerless system that, without using any cleaning device, reuses the transfer residual toner residing on the photosensitive member after the transferring step by removing and recovering the toner from the photosensitive member by performing a “cleaning-simultaneous-with-developing” operation at the developing apparatus.
By performing the cleaning-simultaneous-with-developing operation, the transfer residual toner on the photosensitive member after the transferring step is recovered at the developing apparatus during the following developing steps. That is, this is a method with which the photosensitive member, onto which the transfer residual toner adheres, is continuously charged and exposed during the formation of an electrostatic latent image. In a step for developing this electrostatic latent image, there is applied a fog removal bias (fog removing potential difference Vback that is a potential difference between a direct-current voltage applied to the developing apparatus and the potential of the surface of the photosensitive member). As a result, residual toner on the photosensitive member surface that exists on each portion (non-image portion) that should not be developed is removed and recovered at the developing apparatus.
With this system, the transfer residual toner is recovered at the developing apparatus and is reused for the developing of an electrostatic latent image in the following steps. Consequently, it becomes possible to prevent the generation of waste toner and also to reduce the burden during maintenance work. Also, there is not used any cleaner, so that this system is advantageous when an image forming apparatus is miniaturized.
In an image forming apparatus that uses the cleanerless system adopting the cleaning-simultaneous-with-developing method described above, in the case where a contact electrifying device that electrifies the surface of the photosensitive member while being abutted against the photosensitive member is used as an electrifying device, there is a case where toner, out of transfer residual toner on the photosensitive member, that has an electrification polarity reversed to a polarity opposite to a normal polarity adheres to the contact electrifying device while the transfer residual toner is passing through a contact nip portion (electrifying portion) between the photosensitive member and the contact electrifying device. As a result, this phenomenon may cause a situation where the contact electrifying device is polluted with toner at a level exceeding a permissible level and becomes a cause of poor electrification.
That is, toner having an electrification polarity that is inherently reversed to a polarity opposite to the normal polarity coexists in toner functioning as developer although the amount of such toner is small. Also, even among toner whose electrification polarity is the normal polarity, there exists toner, whose electrification polarity is reversed as a result of the influence of a transferring bias or separation discharge, or toner whose electric charge amount is reduced as a result of diselectrification.
As a result, the transfer residual toner contains toner whose electrification polarity is the normal polarity, toner whose electrification polarity is reversed to the opposite polarity, and toner whose electric charge amount is small. The reversed toner and toner with a small electric charge amount in the transfer residual toner tend to adhere to the contact electrifying device while passing through the contact nip portion (electrifying portion) between the photosensitive member and the contact electrifying device.
Also, in order to remove and recover the transfer residual toner on the photosensitive member through the cleaning-simultaneous-with-developing operation, it is required that the electrification polarity of the transfer residual toner on the photosensitive member that passes through the electrifying portion and is carried to the developing portion is the normal polarity and its electric charge amount is the electrification amount of toner that is possible to develop an electrostatic latent image on the photosensitive member by the developing apparatus. It is impossible to remove and recover the reversed toner or the toner having an inappropriate electric charge amount from the photosensitive member into the developing apparatus, so that there is a fear that such toner becomes a cause of faulty images.
Even if transfer residual toner that exists on the photosensitive drum and is carried from the transferring portion to the electrifying portion contains toner having an electrification polarity that is the normal polarity, toner having an opposite polarity, and toner having a small electric charge amount, it is possible to prevent the adhesion of the transfer residual toner to the contact electrifying device by aligning the electrification polarities of the toner with the normal polarity through electrification to the normal polarity and unifying the electric charge amounts of the toner using a toner electrification amount control means.
However, the transfer residual toner electrified by the toner electrification amount control means for the sake of preventing the toner adhesion to the contact electrifying device has an electric charge amount that is larger than that of toner that is capable of developing an electrostatic latent image on the photosensitive member, so that it is difficult to remove and recover the transfer residual toner through the cleaning-simultaneous-with-cleaning operation at the developing apparatus. Consequently, there is a fear that faulty images are generated because the toner residing on the photosensitive member is superimposed on the next image.
In view of this problem, conventionally, a toner electrification amount control means for electrifying the transfer residual toner on the photosensitive member is provided at a position that is on the upstream side of the contact electrifying device and on the downstream side of the transferring means in a direction in which the photosensitive member moves. In addition, a transfer residual toner unifying means that unifies the transfer residual toner is provided at a position that is on the upstream side of the toner electrification amount control means and on the downstream side of the transferring means. By applying constant direct-current voltages to these toner electrification amount control means and the transfer residual toner unifying means, there is solved the problem described above.
That is, the residual toner residing on the photosensitive member after the transferring is unified by the transfer residual toner unifying means and the unified transfer residual toner on the photosensitive member is electrified to have the normal polarity by the toner electrification amount control means. Following this, simultaneously with the electrification of the surface of the photosensitive member by the contact electrifying device, the transfer residual toner electrified by the toner electrification amount control means is electrified to have an electric charge amount that is proper for the removal and recovery by the cleaning-simultaneous-with-developing operation at the developing apparatus. In this manner, the transfer residual toner is recovered at the developing apparatus.
However, in accordance with the diversification of user's needs in recent years, there occurs a case where a large quantity of transfer residual toner is generated at a time as a result of an operation for successively forming images, such as photographic images, having high printing ratios (image ratios or coverage ratios), a system that performs multiplex developing on a photosensitive member in an image forming apparatus that is capable of forming a color image, and the like.
In this case, there occurs the adhesion of the transfer residual toner to the contact electrifying device and the following rotation of the transfer residual toner on the photosensitive member due to the poor removal and recovery in the developing apparatus. As a result, there is a case where faulty images are generated due to poor electrification and the like.